doc: west: remove some unnecessary 'master' usages

Although a project's default revision is still "master", we can change
some examples around to refer to "main" branches instead, in keeping
with the changes we're making for Zephyr more generally.

We cannot change the default project revision away from "master" in
west v0.10.x, because that would be a schema change. That will have to
wait for v0.11.0.

However, in the meantime, we are making some inclusive language
changes related to 'west init', because the --mr option is not
governed by a schema. These will be documented in the next commit.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>